Success!
In an earlier post, I mentioned that I had tweaked the .bmp file a couple/few times. I did that because when I downloaded it to HC1 and viewed it on HC1, I found the image to be too, dark, too low, too big, etc. Each time I revised the .bmp file I just saved it with the same file name and then downloaded that same file name to HC1.
I also only ever used HC1 to load it to the LM-3S module.
RE's first recommendation was to 1) delete/remove SP 8306 from HC2, 2) use HC1 to re-select the loco image for SP 8306 and save it, and then 3) re-acquire (find product) SP 8306 on HC2. I did this and had what I would call a partial success. The loco image for SP 8306 on HC2 finally changed from the standard, blue RE loco to my custom picture of an SP SD40T-2, but it was of an earlier version that I didn't like. Huh!? The two HC's were showing two different images for the same LM-3S module! All I can figure is that somehow there was an artifact in the file (because of the numerous changes I made to it) that caused this to happen. Remember, I never downloaded the image to HC2 from RailPro Assistant. HC2 only got that image via/through the LM-3S module. Strange huh?
So, RE finally suggested 1) I remove/delete SP 8306 from HC1, 2) save the final version of my .bmp with a new name, 3) download that to HC2 (not HC1), 4) use HC2 to copy it to SP 8306's LM-3S module and then 5) "find" SP 8306 on HC1.
Voila! That worked!
Lesson for the day - anytime you revise a loco picture/image file, DON'T simply save it with the same file name and upload that to the HC, label them something like PICTURE 1, PICTURE 2, PICTURE 3, etc., so that the HC will have distinctly different files to utilize.
Whew - glad that's over!
P.S. I wish I knew who I was conversing with from RE so as to thank them. They never "signed" their name on any of the emails...